EP0201841B1 - Process for phosphating metal surfaces - Google Patents

Abstract

A solution for conditioning a metal surface before phosphate conversion coating processes comprises metallic titanium, pyrophosphate and water soluble anionic organic compound.

Description

The invention relates to a method for phosphating metal surfaces, in which the metal surfaces are brought into contact with an aqueous pre-rinse bath containing titanium, phosphate and pyrophosphate as activating agent before they are phosphated.

Activation of a metal surface before phosphating means contacting this surface with an aqueous pre-rinse bath in which an activating agent is extremely finely dispersed as a solid phase. By activating such a treatment, the number of phosphate crystals growing per unit metal surface during the phosphating is significantly increased. At the same time, the size of the individual crystals in the finished phosphate layer is in some cases considerably reduced, the basis weight of the phosphate layer is reduced and the time required for covering the metal surface with phosphate crystals is shortened. The interpretation of this effect is that the treatment with activating agent also fixes crystal nuclei on the metal surface, from which the growth of the phosphate crystals starts during the phosphating.

A number of substances have become known as activating agents, of which finely dispersed titanium phosphate plays a special role (US Pat. No. 2,310,239, EP Pat. No. 56,675, EP Pat. No. 131,298, DE Pat. No. 20 38 105, GB). A - 2 136 454).

The effectiveness of the activation extends both to the phosphating of iron and steel and to that of non-ferrous metals, for example zinc, zinc alloys, metals coated with zinc and zinc alloys, aluminum, aluminum alloys and metals coated with aluminum and its alloys.

In the practical application of the pre-rinse baths, however, it was found that a coarsening of the phosphate crystals and an increase in the layer weight of 25 to 50%, possibly even more, can be determined just a few days after the preparation. The corrosion-protecting effect of the phosphate layer and the layer uniformity can also deteriorate considerably. The aforementioned reduction in effectiveness occurs regardless of whether the pre-rinse baths are used for activation purposes or not. It is therefore obviously the result of an "aging" of the baths.

In order to achieve an approximately perfect activation, the only option so far was to replace part of the pre-rinse bath with a freshly prepared bath. However, the results were not clearly certain here either. Most of the time, therefore, it was decided to make a new start.

One proposal to switch off aging as such is to add montmorillonite, in particular in the form of bentonite, to the pre-rinse bath (EP-A-117 599). However, this proposal did not meet expectations in large-scale operation.

The object of the invention is to provide a method for phosphating metal surfaces by pretreatment to be provided by means of a pre-rinse bath which does not have the known, in particular the aforementioned disadvantages, or has them in a greatly reduced form, can be used without additional work and remains effective over a considerably extended period of time.

enthält und einen pH-Wert von 8 bis 9,5 aufweist.The object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that the metal surfaces are brought into contact with a prewash bath which

contains and has a pH of 8 to 9.5.

With the aid of the method according to the invention, it is possible to produce dense phosphate coatings with an increase in the layer weight of at most 15% by weight even during a comparatively long service life. In addition, it was found that far less high-quality water can be used for the preparation of the pre-rinse bath than was previously the case.

With regard to the concentration of the active ingredients, the following was found.

If the titanium content is less than 3 ppm, the phosphate layer produced becomes less dense. If the concentration is increased above 100 ppm, the phosphate layer remains as dense as in the range from 3 to 100 ppm, but no additional effect is obtained, so that such high concentrations are not recommended from an economic point of view.

If the pyrophosphate concentration is reduced below 60 ppm, the phosphate layer becomes less dense and its weight increases. The well-known aging effect is noticeable. When the concentration exceeds 360 ppm, it becomes increasingly difficult to form a zinc phosphate type layer; instead, an iron phosphate layer is increasingly formed. Layers with the desired density of the phosphate crystals and the desired low layer weight are obtained only in the range from 60 to 360 ppm, which should be explained by the suppression of the coagulation of the titanium colloids.

If the lower limit for the total phosphate content (calculated as PO₄) is undershot, the titanium colloids tend to coagulate and thus to the aging effects already explained above. Exceeding the total phosphate content of 3000 ppm by orthophosphate does not bring any disadvantages, but - taking economic aspects into account - no advantages either.

The fourth essential component of the pre-rinse bath - especially in connection with pyrophosphate - ensures that perfect layers are obtained even after relatively long periods of use. The reason for this is likely to be that the coagulation of the titanium colloids responsible for aging is suppressed or the titanium colloids are stabilized. This stabilization could possibly also be achieved by correspondingly high amounts of pyrophosphate, but then the metal surfaces become inactive and the layer formation is inhibited. The combined use of both bath components in the concentrations mentioned ensures that on the one hand the initially present stability of the titanium colloids and at the same time the activity of the metal surfaces is retained.

Compliance with the pH range is important insofar as there is a tendency for the titanium colloids to coagulate when the value falls below and the conditioning effect of the pre-rinse bath is lost when it is exceeded.

Particularly advantageous results are achieved if, according to an advantageous embodiment of the invention, the metal surfaces are brought into contact with a pre-rinse bath whose water-soluble, anionic polymer is formed from the unsaturated carboxylic acid, acrylic acid, metacrylic acid and / or maleic acid.

The invention is illustrated by the following examples, for example and in more detail.

Examples

• 1. Reinigung mit einem alkalischen Reiniger bei 40 ± 1°C während 180 sec im Tauchen (pH = 10,2, Gesamtalkalinität 16 ± 1 Punkte bei Titration einer 10 ml-Badprobe gegen Bromphenolblau mit 0,1 n Schwefelsäure).
• 2. Wasserspülung
• 3. Aktivierungsbehandlung mit dem Vorspülbad im Tauchen bei Raumtemperatur während 30 sec, und zwar unmittelbar nach dem Ansatz und nach 10-tägiger Wartezeit (Zusammensetzung des Vorspülbades gemäß Tabellen 1 und 2, wobei für die Vorspülbäder gemäß Tabelle 1 Leitungswasser mit einer spezifischen Leitfähigkeit von 150 µS/cm, für Vorspülbäder gemäß Tabelle 2 Leitungswasser mit einer spezifischen Leitfähigkeit von 600 µS/cm verwendet wurde.)
• 4. Phosphatierung mit einer nitritbeschleunigten Zinkphosphatlösung im Tauchen bei 43 ± 1°C während 120 sec (freie Säure 0,8 bis 1,0 Punkte bei Titration einer 10 ml-Badprobe mit 0,1 n NaOH gegen Bromphenolblau; Gesamtsäure 22 bis 24 Punkte bei Titration einer 10 ml-Badprobe mit 0,1 n NaOH gegen Phenolphthalein, Nitritpunkte 2,5 bis 3 nach der Saccharometermethode ermittelt).
• 5. Wasserspülung
• 6. Spülung mit entsalztem Wasser
• 7. Trocknung bei 100°C

Steel sheets of quality JIS-G-3141, SPCC, were treated according to the following procedure:

• 1. Cleaning with an alkaline cleaner at 40 ± 1 ° C for 180 sec in immersion (pH = 10.2, total alkalinity 16 ± 1 points when titrating a 10 ml bath sample against bromophenol blue with 0.1 N sulfuric acid).
• 2. Water rinse
• 3.Activation treatment with the pre-rinse bath in immersion at room temperature for 30 seconds, immediately after the preparation and after a 10-day waiting period (composition of the pre-rinse bath according to Tables 1 and 2, tap water with a specific conductivity of 150 for the pre-rinse baths according to Table 1) µS / cm, for pre-rinse baths according to Table 2 tap water with a specific conductivity of 600 µS / cm was used.)
• 4. Phosphating with a nitrite-accelerated zinc phosphate solution while diving at 43 ± 1 ° C for 120 sec (free acid 0.8 to 1.0 points when titrating a 10 ml bath sample with 0.1 N NaOH against bromophenol blue; total acid 22 to 24 points when titrating a 10 ml bath sample with 0.1 N NaOH against phenolphthalein, nitrite points 2.5 to 3 determined using the saccharometer method).
• 5. Water rinse
• 6. Rinse with demineralized water
• 7. Drying at 100 ° C

The phosphate layers formed were evaluated according to the appearance
with o for a dense and even layer,
with x for an uneven and incomplete layer with rust film,
based on the layer weight, determined from the weight difference before and after removal of the phosphate layer with 5% by weight chromic acid solution,
according to the size of the crystals (measured with an electron microscope model ZSM-T100 from Nihon Denski KK)

The test results are summarized in Tables 1 and 2. This shows that the layers produced by the process according to the invention (Table 1 Examples 1 to 8; Table 2 Examples 1 to 8) are of excellent quality in terms of appearance, weight gain and crystal size, while the layers produced by the known process (Table 1 Comparative Example 1 and Table 2 Comparative Example 1) also show good results immediately after the preparation, but are unsatisfactory after a waiting period of 10 days. The results of comparative experiments 2 and 3 also show that not only the activation treatment after a waiting period of 10 days (calculated from the time of the preparation of the pre-rinse bath), but also leads to unusable phosphate layers due to the too high content of copolymer and condensed naphthalenesulfonate with freshly added pre-rinse baths.

Table 2 also shows the results obtained with essentially the same pre-rinse baths in Table 1. Only the water used had a specific conductivity of 150 µS / cm instead of 600 µS / cm. The effective bath components and their amounts were the same. From this it follows that the lower water quality has far less impact on the results obtained than in the known pre-rinse baths (cf. Experiment 1).

As a result it follows that even after a comparatively long period since the pre-rinse bath was applied, layers of substantially the same quality as those at the time of the new preparation are obtained with the method according to the invention. In constant use of the process, a complete or partial replacement of the pre-rinse bath with a freshly prepared bath - measured using conventional processes - is only necessary after much longer periods of time.

Claims (2)

1. A process for the phosphate coating of metal surfaces, wherein prior to their phosphate coating the metal surfaces are brought into contact with an aqueous preliminary rinsing bath containing as activating agents titanium, phosphate and pyrophosphate, characterized in that the metal surfaces are brought into contact with a preliminary rinsing bath which contains

   

   and has a pH of 8 to 9.5.
2. A process according to claim 1, characterized in that the metal surfaces are brought into contact with a preliminary rinsing bath whose water-soluble anionic copolymer is formed by the unsaturated carboxylic acid, acrylic acid, methacrylic acid and/or maleic acid.